My invention relates generally to support structures and, more particularly, to a novel and useful structural support assembly for use in permafrost areas or in any areas having active ground layers subject to a severe annual freeze-thaw cycle.
Permafrost is material which is largely frozen permanently. It is usually a mixture of soil, rock and ice although it can be anything from solid rock to muddy ice. In the arctic regions, permafrost may extend from a few feet to hundreds of feet below the surface. The permafrost is separated from the surface by an upper layer (the tundra) and its surface vegetation. The upper layer or tundra serves as insulation to limit permafrost thaw in the summer but is subject to a seasonal freeze-thaw cycle. The permafrost thaw in the summer, however, can create an unstable condition for structures constructed in permafrost areas. This is, of course, more so in wet, ice-rich, permafrost areas than in dry, stable, permafrost areas of well drained soil or rock.
There are severe problems associated with support and stabilization of structures in the arctic regions where permafrost is prevalent. Alaskan railroads, for example, require the expenditure of thousands of dollars each year to repair soil slippages and track roughness resulting from the annual freeze-thaw cycle and disturbances of the ground cover by the intrusion of man and his machines. When the tundra is broken or removed, the permafrost looses its insulation and begins to melt and erode. Thus, tracks left by a tractor or caterpillar train can become a deep ditch and alter the surface drainage pattern over a wide area.
In cities and regions which overlay permafrost areas, a gravel insulating technique is generally used in construction over such areas. A raised gravel pad, for example, is ordinarily employed to provide a suitable support or work area on permafrost. Foundation structures embedded in permafrost are also commonly surrounded completely by a layer of insulating gravel. In areas of ice-rich permafrost and/or during a strong summer thaw, however, even the use of a relatively thick insulating gravel layer is inadequate to prevent some subsidence and possibly accompanying damage of the supported structure or apparatus. On the other hand, instead of subsiding, support posts or poles for arctic overhead communications and power lines have presented a particular problem with "pole jacking" wherein the annual seasonal uplift due to frost heave can actually lift the poles and their anchors completely out of the ground. The pole jacking problem has plagued all of the utility companies throughout vast areas of the subarctic regions, and is presently considered to have no reasonable economic solution.
The U.S. Pat. No. 3,217,791 of Erwin L. Long on Means for Maintaining Permafrost Foundations patented Nov. 16, 1965 discloses and claims a thermo-valve foundation system including a tubular container partially filled with a low boiling point liquid, either propane or carbon dioxide, and a layer of gravel completely surrounding its lower portion. The thermo-valve tubular container operates during periods of subfreezing temperatures to absorb heat from the adjoining permafrost, to freeze the adjacent unfrozen soil and increase its strength of adhesion to the foundation. The container itself serves as a foundation piling or support pole which is used with a gravel layer completely surrounding its lower portion. It is, however, not only costly but frequently impractical and infeasible to provide a sufficiently large and thick insulating gravel layer entirely around and below the lower portion of each pole to stabilize it. Moreover the metallic tubular container of the thermo-valve system is obviously limited by practical considerations in height or length and location whereas a wooden utility pole of any substantial height or length can be economically used in any location.